Grad Profile

PhD student tracks Cambrian fossils

PhD student Sean Robson.

Photo by Joel Deshaye

By Joel Deshaye

Since Sean Robson began studying Cambrian fossils, he has collected thousands of kilograms of ancient rocks. The rocks are usually around 500 million years old and hopefully contain fossilized brachiopods such as the Linguliformea, Robsons specialty.

Robson is a PhD student in the Geological Sciences program at the U of S. His B.Sc. was in the universitys Paleobiology program, which involves biology, anthropology, and geology. It is the only program of its kind in Canada.

The Linguliformea are tiny marine invertebrates that live in their own bi-valved shells.

"Theyre usually about a millimetre in size," says Robson. "Theyre difficult to collect because you dont know theyre here," he says, referring to the rock in his hand  a 450-million-year-old specimen that probably has many brachiopods in it.

"My very first year of field work I collected 700 kilograms of rock and there wasnt a thing in them. Second time out I collected about 2,000 kilograms of rock. Based on speculation I collected a whole lot to dissolve it all." It took more than two years to dissolve all the limestone using acetic acid, which dissolves calcium carbonate but not the phosphate in brachiopod shells.

Under a microscope, one can see the circular scars where internal muscles were attached to the shells. The shells can be as large as a few millimetres in diameter or almost as small as a grain of sand. "They can have interior tubes; they can have spines on the outside. Theres an amazing diversity of form."

The Linguliformea have been neglected because theyre hard to study and no one knew how to use them for reliably dating rocks. To date rocks, says Robson, "what you need is a rapid species change over a short period of time, so then you can identify the age of a rock based on the species present." Only when the Linguliformea were studied from inside their shells did they reveal significant evolution.

Robson hopes his research will reveal more about paleo-oceanic currents, the brachiopods larval distribution patterns, and even the position of the Paleozoic continents.

In the Cambrian period, Sas-katchewan and Australia were part of two large separate land masses on opposite sides of the world. However, brachiopods here are intriguingly similar to the brachiopod fossils found in Australia today.

Brachiopod larvae can swim until their shells develop and they sink to the ocean floor. "The only way they could cross water is to go from one shallow area to another shallow area in the time frame of their larval stage. If the continents are too far apart they wont be able to travel between them in, say, two months time," says Robson. But since it is fairly certain that the continents were at opposite sides of the globe, "something else is going on."

Robson speculates that the Linguliformea stuck to the sides of trenches in deeper waters, feeding on the sulphides and other nutrients rising from the magma. Ocean currents would run along these trenches, "like an oceanic superhighway," Robson suggests. "It might be along these routes that they could travel around the world."

Robson hopes to trace the progress of the creatures along the ocean ridges. "Thats what so exciting about these. Theres so much they can tell us. Not only about dating the rocks but about paleo-oceanography, paleoenvironment, and also the ecology of the sea floor."

Robson predicts "a renaissance in the study of linguliform brachiopods."

One of his early inspirations was the book Wonderful Life, by Stephen Jay Gould. It sparked Robsons interest in paleobiology and he went to see Dr. Brian Pratt at the U of S. "I told him I want to study some Cambrian invertebrates. Hes a trilobite researcher himself and had been interested in the potential of these brachiopods because he finds them in Cambrian rocks all the time."

Since there are so few experts in his field, Robson felt some initial frustration. Fortunately, he says that "the brachiopod community  the community of researchers, not the community of brachiopods  is very close-knit."

Now he enjoys his research very much. "Its fascinating. Its a lot of fun. One of the best things about it is the opportunity to travel and get out into the field a lot. Youre not just stuck in the lab all the time ... you never get bored. Its never the same thing twice."

Joel Deshaye writes profiles of U of S graduate students as part of a fellowship with the College of Graduate Studies and Research.